Bandwagon fans are arguably the most annoying aspect of sports—they simply go from team to team depending on who's winning or who's popular.
Having bandwagon fans as a part of your fanbase does not demean any of the true fans—in fact, most of the biggest bandwagon fanbases also have large amounts of true fans. It's not the true fans' faults that their team is so well liked.
Teams who win consistently or have a star player tend to have more bandwagon fans than teams who don't win on a regular basis.
There are no stats to back this up; it is simply my opinion derived from my observations across the NFL.
Here are the 16 biggest bandwagon fanbases in the NFL.